Software adjustments were successfully completed for the millimeter wave scanners, but not the backscatter devices, according to the Transportation Safety Administration. The decision alleviates concerns about excess radiation exposure that might have occurred if the machines malfunctioned.

Four radiation scientists from academic institutions sent a letter of concern in 2010 to the White House questioning “the extent to which the safety of this scanning device has been adequately demonstrated,” especially for frequent fliers who are going through them on a daily basis. And the European Union opted to use millimeter wave scanners in its airports after potential risks became known.

A TSA contractor who spoke with the Washington Post on the condition of anonymity speculated that concerns about radiation risks contributed to the decision to remove the machines from US airports. I suspect he’s right, though TSA officials insist that radiation safety wasn’t a factor.